Method for removing 6-aminocapronitrile from mixtures that contain 6-aminocapronitrile, adipodinitrile and hexamethylenediamine

ABSTRACT

The invention relates to a method for removing, by distillation, 6-aminocapronitrile from mixtures that contain 6-aminocapronitrile, adipodinitrile and hexamethylenediamine, by a) removing the hexamethylenediamine from the mixture while obtaining a mixture (I) that has a hexamethylenediamine content of less than 1 wt. -%, b) removing completely or partially the 6-aminocapronitrile from mixture (I) while obtaining a mixture (II) whose content in substances that have a higher boiling point as 6-aminocapronitrile under distillation conditions and that cannot be formed by dimerization reactions when 6-aminocapronitrile is thermally treated is less than 1 wt. -%, and c) completely or partially removing from mixture (II) the hexamethylenediamine that might be present while obtaining a mixture (IV) whose hexamethylenediamine content is higher than that of mixture (II), and a mixture (V) whose hexamethylenediamine content is lower than that of mixture (II).

The present invention relates to a process for the distillativeseparation of 6-aminocapronitrile from mixtures containing6-aminocapronitrile, adipodinitrile and hexamethylenediamine, wherein

-   a) the hexamethylenediamine is separated from the mixture to give a    mixture (I) with a hexamethylenediamine content of less than 1% by    weight;-   b) all or part of the 6-aminocapronitrile is separated from the    mixture (I) to give a mixture (II) whose content of substances which    boil above 6-aminocapronitrile under distillation conditions and    cannot be formed by dimerization reactions of 6-aminocapronitrile    under thermal stress is less than 1% by weight; and-   c) all or part of the hexamethylenediamine present is separated from    the mixture (II) to give a mixture (IV) whose hexamethylenediamine    content is higher than that of the mixture (II), and a mixture (V)    whose hexamethylenediamine content is lower than that of the mixture    (II).

It is known that 6-aminocapronitrile can be used for the preparation ofcaprolactam or polyamides. The 6-aminocapronitrile must have a highpurity for such purposes.

6-Aminocapronitrile is conventionally prepared by the partial catalytichydrogenation of adipodinitrile, which can be obtained by the doublehydrocyanation of butadiene in the presence of catalysts. This normallygives a mixture containing 6-aminocapronitrile, unreacted adipodinitrileand hexamethylenediamine, optionally together with solvents, substancesboiling below hexamethylenediamine, substances boiling aboveadipodinitrile, and other by-products.

Numerous processes for the separation of 6-aminocapronitrile from suchmixtures to give a 6-aminocapronitrile of said high purity are known,for example from WO 96/20931 and WO 97/23454.

The disadvantage of these processes is the high energy expenditurerequired to recover 6-aminocapronitrile of sufficient purity from themixture.

It is an object of the present invention to provide a process whichmakes it possible, in a technically simple and economic manner andavoiding said disadvantages, to recover 6-aminocapronitrile ofsufficient purity from mixtures containing 6-aminocapronitrile,adipodinitrile, hexamethylenediamine and optionally solvents and othersecondary components.

We have found that this object is achieved by the process defined at theoutset.

Processes for the preparation of mixtures containing6-aminocapronitrile, adipodinitrile, hexamethylenediamine and optionallysolvents, substances boiling below hexamethylenediamine, substancesboiling above adipodinitrile, and other by-products are known per se.

Thus mixtures containing 6-aminocapronitrile, adipodinitrile andhexamethylenediamine can be obtained by the partial catalytichydrogenation of adipodinitrile.

Based on the sum of 6-aminocapronitrile, adipodinitrile andhexamethylenediamine, such mixtures conventionally contain from 5 to 90%by weight, preferably from 10 to 80% by weight and particularlypreferably from 20 to 70% by weight of 6-aminocapronitrile and from 5 to90% by weight, preferably from 10 to 80% by weight and particularlypreferably from 20 to 70% by weight of hexamethylenediamine, theremainder being unreacted adipodinitrile.

In the light of previous observations, the catalyst used for thehydrogenation is not critical in terms of the process according to theinvention. The mixture can be separated from the catalyst after thehydrogenation in a manner known per se, for example by filtration, thisbeing preferred in the case of a suspension hydrogenation, or by removalof the mixture from the reaction vessel to leave the catalyst in thereaction vessel, this being preferred in the case of a fixed bedhydrogenation.

The hydrogenation can advantageously be carried out in the presence of asolvent. Suitable solvents are organic solvents such as alcohols,preferably alkanols and especially C₁–C₄ alkanols like methanol,ethanol, n-propanol, i-propanol, n-butanol, i-butanol or s-butanol,esters, preferably esters of an alkanecarboxylic acid, especially aC₁–C₄ alkanecarboxylic acid like formic acid, acetic acid, propionicacid or butyric acid, with an alkanol, especially a C₁–C₄ alkanol likemethanol, ethanol, n-propanol, i-propanol, n-butanol, i-butanol ors-butanol, ethers such as linear or cyclic ethers, for example dimethylether, diethyl ether, methyl t-butyl ether or tetrahydrofuran,hydrocarbons such as aliphatic or aromatic hydrocarbons, for examplebenzene, toluene, ethylbenzene, o-xylene, m-xylene or p-xylene, aminessuch as primary, secondary or tertiary amines, lactones such asbutyrolactone, lactams such as caprolactam, pyrrolidone orN-methylpyrrolidone, and amides, or inorganic solvents such as ammonia,or mixtures thereof.

Preferred solvents are alcohols, especially methanol and ethanol,aromatic hydrocarbons, especially toluene, and ammonia, or mixturesthereof.

If the hydrogenation is carried out in the presence of a solvent, thereaction mixture will then contain this solvent. Prior to the processaccording to the invention, the solvent can advantageously be separatedfrom the reaction mixture in a manner known per se, for example bydistillation or rectification.

This separation can advantageously be performed by fractionaldistillation in one or more, such as 2 or 3, distillation apparatuses.

Suitable apparatuses are those conventionally used for distillation, forexample those described in: Kirk-Othmer, Encyclopedia of ChemicalTechnology, 3rd ed., vol. 7, John Wiley & Sons, New York, 1979, pages870–881, such as sieve-plate columns, bubble-cap columns or packedcolumns, with or without a side discharge.

By-products, for example substances boiling below hexamethylenediamineand/or substances boiling above adipodinitrile, can be formed in thehydrogenation.

In terms of the present invention, substances boiling belowhexamethylenediamine are understood as meaning compounds which have aboiling point below that of hexamethylenediamine under the appropriatedistillation conditions.

Examples of possible substances boiling below hexamethylenediamine arehexamethyleneimine, hexylamine, aminomethylcyclopentylamine anddiaminocyclohexane. Such compounds are present in the mixtures obtainedfrom the hydrogenation in overall amounts of up to 10% by weight,preferably of up to 5% by weight, based on the sum of6-aminocapronitrile, adipodinitrile and hexamethylenediamine.

Prior to the process according to the invention, substances boilingbelow hexamethylenediamine can advantageously be separated from thereaction mixture in a manner known per se, for example by distillationor rectification.

This separation can advantageously be performed by fractionaldistillation in one or more, such as 2 or 3, distillation apparatuses.

Suitable apparatuses are those conventionally used for distillation, forexample those described in: Kirk-Othmer, Encyclopedia of ChemicalTechnology, 3rd ed., vol. 7, John Wiley & Sons, New York, 1979, pages870–881, such as sieve-plate columns, bubble-cap columns or packedcolumns, with or without a side discharge.

Any solvent used and any substances present which boil belowhexamethylenediamine can be separated off simultaneously in step a orbeforehand. Preferably, any solvent used is separated off first (stepa0), followed by any substances present which boil belowhexamethylenediamine (step a1).

Step a1 should advantageously be carried out at a pressure below ambientatmospheric pressure, preferably at an absolute pressure below 500 mbar.

According to the invention, hexamethylenediamine is separated from themixture in step a to give a mixture (I) with a hexamethylenediaminecontent below 5% by weight, preferably below 1% by weight, based on themixture (I).

This separation can advantageously be performed by fractionaldistillation in one or more, such as 2 or 3, distillation apparatuses.

Suitable apparatuses are those conventionally used for distillation, forexample those described in: Kirk-Othmer, Encyclopedia of ChemicalTechnology, 3rd ed., vol. 7, John Wiley & Sons, New York, 1979, pages870–881, such as sieve-plate columns, bubble-cap columns or packedcolumns, with or without a side discharge, preferably with a sidedischarge.

Step a should advantageously be carried out at a pressure below ambientatmospheric pressure, preferably at an absolute pressure below 500 mbar.

In one preferred embodiment, hexamethylenediamine and components boilingbelow hexamethylenediamine can be separated off together, i.e. steps a1and a can be carried out together.

In this operation, hexamethylenediamine can advantageously be recoveredat a side discharge.

In one particularly preferred embodiment, the column used has aseparating plate in the region between the feed and the side discharge.

According to the invention, all or part of the 6-aminocapronitrile isseparated from the mixture (I) in step b to give a mixture (II) whosecontent of substances which boil above 6-aminocapronitrile underdistillation conditions and cannot be formed by dimerization reactionsof 6-aminocapronitrile under thermal stress is less than 2% by weight,preferably less than 0.1% by weight.

This separation can advantageously be performed by fractionaldistillation in one or more, such as 2 or 3, distillation apparatuses.

Suitable apparatuses are those conventionally used for distillation, forexample those described in: Kirk-Othmer, Encyclopedia of ChemicalTechnology, 3rd ed., vol. 7, John Wiley & Sons, New York, 1979, pages870–881, such as sieve-plate columns, bubble-cap columns or packedcolumns, with or without a side discharge, preferably with a sidedischarge.

Step b should advantageously be carried out at a pressure below ambientatmospheric pressure, preferably at an absolute pressure below 500 mbar.

According to the invention, all or part of the hexamethylenediaminepresent is separated from the mixture (II) in step c to give a mixture(IV) whose hexamethylenediamine content is higher than that of themixture (II), and a mixture (V) whose hexamethylenediamine content islower than that of the mixture (II).

This separation can advantageously be performed by fractionaldistillation in one or more, such as 2 or 3, distillation apparatuses.

Suitable apparatuses are those conventionally used for distillation, forexample those described in: Kirk-Othmer, Encyclopedia of ChemicalTechnology, 3rd ed., vol. 7, John Wiley & Sons, New York, 1979, pages870–881, such as sieve-plate columns, bubble-cap columns or packedcolumns, with or without a side discharge.

Step c should advantageously be carried out at a pressure below ambientatmospheric pressure, preferably at an absolute pressure below 500 mbar.

In one preferred embodiment, the pressure in step a should be lower thanthe pressure in step c.

In another preferred embodiment, the mixture (IV) obtained in step c canbe recycled into step a, the mixture (IV) preferably being vaporous.

In another embodiment of step b in terms of the present invention, it ispossible to allow a higher content of substances boiling above6-aminocapronitrile to give a mixture (IIa) and separate componentsboiling above 6-aminocapronitrile from the mixture (IIa) to give amixture (VI) whose 6-aminocapronitrile content is higher than that ofthe mixture (II), and a mixture (VII) whose 6-aminocapronitrile contentis lower than that of the mixture (II).

In another preferred embodiment, the mixture (VII) can be recycled intostep b.

The hexamethylenediamine content of the mixture (V) shouldadvantageously be at most 5000 ppm, preferably 0 to 1000 ppm andparticularly preferably 0 to 200 ppm, based on the weight of the mixture(V). The sum of the components of the mixture (V) other than6-aminocapronitrile should advantageously be at most 5000 ppm,preferably 0 to 500 ppm and particularly preferably 0 to 200 ppm, basedon the weight of the mixture (V).

The total energy consumption of the process according to the invention,calculated as the sum of the energy consumptions in steps a, b and c,and optionally a0 and a1, is lower than the energy consumption requiredwhen the separation is carried out in only two steps a and c, andoptionally a0 and a1, to achieve the same hexamethylenediamine contentin the 6-aminocapronitrile separated off.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 schematically illustrate the separation of a raw mixturecomprising 6-aminocapronitrile (ACN), adipodinitrile (ADN) andhexamethylenediamine (HMD), as well as substances boiling aboveadipodinitrile (HB) and substances boiling below hexamethylenediamine(LB).

DETAILED DESCRIPTION OF THE DRAWINGS

In the embodiment of the process which is illustrated in FIG. 1, the rawmixture comprising 6-aminocapronitrile (ACN), adipodinitrile (ADN) andhexamethylenediamine (HMD), as well as substances boiling aboveadipodinitrile (HB) and substances boiling below hexamethylenediamine(LB) is separated in a distillation column (K1) into ahexamethylenediamine (HMD) fraction which comprises substances boilingbelow hexamethylenediamine (LB), and a mixture (I) which comprises6-aminocapronitrile and adipodinitrile and has a hexamethylenediaminecontent of less than 1% by weight. The mixture (I) is conveyed into adistillation column (K2) and is separated into an adipodinitrilefraction which also comprises substances boiling above adipodinitrile(HB) and a mixture (II) which comprises 6-aminocapronitrile and whichcontains less than 1% by weight of substances which boil above6-aminocapronitrile under distillation conditions and which cannot beformed by dimerization reactions of 6-aminocapronitrile under thermalstress. The mixture (II) is conveyed into a distillation column (K3)where it is separated into a mixture (IV) which has a higherhexamethylenediamine content than the mixture (II), and6-aminocapronitrile (mixture (V)) which has a lower hexamethylenediaminecontent than the mixture (II).

The embodiment of the process which is illustrated in FIG. 2 essentiallycorresponds to the embodiment of the process which is illustrated inFIG. 1. FIG. 2 additionally illustrates that the hexamethylenediamine(HMD) fraction which comprises substances boiling belowhexamethylenediamine (LB) can be removed from the distillation column(K1) at a side discharge, and that a part of the mixture (IV) which isseparated off in column (K3) can be recycled into the column (K1).

In the embodiment of the process which is illustrated in FIG. 3, theseparation of the mixture (I) in the distillation column (K2) isconducted such that a mixture (IIa) is separated off which has a highercontent of components boiling above 6-aminocapronitrile than the mixture(II). The mixture (IIa) is then conveyed into the distillation column(K3) where it is separated into a mixture (IV) which has a higherhexamethylenediamine content than the mixture (II), a mixture (VI) whichhas a higher 6-aminocapronitrile content than the mixture (II), and amixture (VII) which has a lower 6-aminocapronitrile content than themixture (II). In accordance with this embodiment, a part or all of themixture (VII) can be recycled into the column (K2).

EXAMPLE

In a distillation column K1, 200 kg/h of a hydrogenation mixturecontaining 29% of hexamethylenediamine, 42% of 6-aminocapronitrile, 27%of adipodinitrile, 0.5% of low-boiling components and 1.5% ofhigh-boiling components were separated off so that 1.5 kg/h containingessentially low-boiling components were withdrawn at the top.

58 kg/h of hexamethylenediamine with a 6-aminocapronitrile content of110 ppm were withdrawn at a side discharge. 156.5 kg/h with ahexamethylenediamine content of 0.2% were withdrawn at the bottom of thecolumn K1. Said column was operated with 90 kg/h of steam. In a seconddistillation column K2, the bottom product withdrawn from column K1 wasdistilled so that 116.5 kg/h of 6-aminocapronitrile with ahexamethylenediamine content of 0.3% were withdrawn at the top. 58 kg/hwith a 6-aminocapronitrile content of 2% were withdrawn at the bottom.The top product of column K2 was distilled in a third column K3 so that17 kg/h of vapor were withdrawn at the top and recycled into column K1.Column K2 was operated with 30 kg/h of steam. 99.5 kg/h of6-aminocapronitrile with a hexamethylenediamine content of 45 ppm werewithdrawn from column K3 at a side discharge. Column K3 was operatedwith 5 kg/h of steam. The steam consumption of columns K1+K3 was 95kg/h.

COMPARATIVE EXAMPLE

In distillation column K1 according to the Example, 200 kg/h of ahydrogenation mixture containing 29% of hexamethylenediamine, 42% of6-aminocapronitrile, 27% of adipodinitrile, 0.5% of low-boilingcomponents and 1.5% of high-boiling components were separated off sothat 1.5 kg/h containing essentially low-boiling components werewithdrawn at the top.

58 kg/h of hexamethylenediamine with a 6-aminocapronitrile content of115 ppm were withdrawn at a side discharge. 156.5 kg/h with ahexamethylenediamine content of 0.05% were withdrawn at the bottom ofcolumn K1. 140 kg/h of steam were necessary to obtain thishexamethylenediamine content at the bottom of the column. In a seconddistillation column K2, the bottom product withdrawn from column K1 wasdistilled so that 116.5 kg/h of 6-aminocapronitrile were withdrawn atthe top. The hexamethylenediamine content of the top discharge was 800ppm. The column was operated with 30 kg/h of steam.

Despite the fact that the energy consumption of column K1 in theComparative Example (140 kg/h) was higher than the sum of the energyconsumptions in columns K1 and K3 in the Example (95 kg/h overall), theACN purity achieved was markedly poorer in the Comparative Example.

1. A process for the distillative separation of 6-aminocapronitrile froma raw mixture containing 6-aminocapronitrile, adipodinitrile andhexamethylenediamine, wherein a) the raw mixture is separated into ahexamethylenediamine fraction and a mixture (I) which comprises6-aminocapronitrile and adipodinitrile and has a hexamethylenediaminecontent of less than 1% by weight; b) all or part of the6-aminocapronitrile is separated from the mixture (I) to give anadipodinitrile fraction and a mixture (II) which comprises6-aminocapronitrile and which contains less than 1% by weight ofsubstances which boil above 6-aminocapronitrile under distillationconditions and which cannot be formed by dimerization reactions of6-aminocapronitrile under thermal stress; and c) the mixture (II) isseparated into a mixture (IV) which has a higher hexamethylenediaminecontent than the mixture (II), and 6-aminocapronitrile (mixture (V))which has a lower hexamethylenediamine content than the mixture (II). 2.A process as claimed in claim 1 wherein the hexamethylenediaminefraction is recovered from the separation in step (a) at a sidedischarge.
 3. A process as claimed in claim 1 wherein thehexamethylenediamine fraction which is separated off from the rawmixture in step (a) comprises components boiling belowhexamethylenediamine.
 4. A process as claimed in claim 1 wherein all orpart of the mixture (IV) which is separated off in step (c) is recycledinto step (a).
 5. A process as claimed in claim 4 wherein the mixture(IV) which is recycled from step (c) into step (a) is vaporous.
 6. Aprocess as claimed in claim 1 wherein the pressure in step (b) is chosenso that the bottom temperature does not exceed 185° C.
 7. A process forthe distillative separation of 6-aminocapronitrile from a raw mixturecontaining 6-aminocapronitrile, adipodinitrile and hexamethylenediamine,wherein a) the raw mixture is separated into a hexamethylenediaminefraction and a mixture (I) which comprises 6-aminocapronitrile andadipodinitrile and has a hexamethylenediamine content of less than 1% byweight; b) all or part of the 6-aminocapronitrile is separated from themixture (I) to give an adipodinitrile fraction and a mixture (IIa) whichcomprises 6-aminocapronitrile; and c) the mixture (IIa) is separatedinto a mixture (IV) which has a higher hexamethylenediamine content thanthe mixture (IIa), aminocapronitrile (mixture (VI)) which has a higher6-aminocapronitrile content than the mixture (IIa), and a mixture (VII))which has a lower 6-aminocapronitrile content than the mixture (IIa). 8.A process as claimed in claim 7 wherein all or part of the mixture (VII)is recycled into step (b).